To date, most transistors used in power electronic applications have typically been fabricated with silicon (Si) semiconductor materials. Common transistor devices for power applications include Si CoolMOS, Si Power MOSFETs, and Si Insulated Gate Bipolar Transistors (IGBTs). While Si power devices are inexpensive, they suffer from a number of disadvantages, including relatively low switching speeds and high levels of electrical noise. More recently, silicon carbide (SiC) power devices have been considered due to their superior properties. III-N semiconductor devices, such as gallium nitride (GaN) devices, are now emerging as attractive candidates to carry large currents, support high voltages and to provide very low on-resistance and fast switching times.
Most conventional III-N high electron mobility transistors (HEMTs) and related transistor devices are normally on, i.e., have a negative threshold voltage, which means that they can conduct current at zero gate voltage. These devices with negative threshold voltages are known as depletion-mode (D-mode) devices. It is preferable in power electronics to have normally off devices, i.e., devices with positive threshold voltages, that do not conduct substantial current at zero gate voltage, in order to avoid damage to the device or to other circuit components by preventing accidental turn on of the device. Normally off devices are commonly referred to as enhancement-mode (E-mode) devices.
Reliable fabrication and manufacturing of high-voltage III-N E-mode transistors has thus far proven to be very difficult. One alternative to a single high-voltage E-mode transistor is to combine a high-voltage D-mode transistor with a low-voltage E-mode transistor in the configuration of FIG. 1 to form a hybrid device, which can be operated in the same way as a single high-voltage E-mode transistor and in many cases achieves the same or similar output characteristics as a single high-voltage E-mode transistor, shown in FIG. 2. The hybrid device of FIG. 1 includes a high-voltage D-mode transistor 23 and a low-voltage E-mode transistor 22 which optionally can both be encased in a package 10, the package including a source lead 11, a gate lead 12, and a drain lead 13. The source electrode 31 of the low-voltage E-mode transistor 22 and the gate electrode 35 of the high-voltage D-mode transistor 23 are both electrically connected together and can be electrically connected to the source lead 11. The gate electrode 32 of the low-voltage E-mode transistor 22 can be electrically connected to the gate lead 12. The drain electrode 36 of the high-voltage D-mode transistor 23 can be electrically connected to the drain lead 13. The source electrode 34 of the high-voltage D-mode transistor 23 is electrically connected to the drain electrode 33 of the low-voltage E-mode transistor 22.
As used herein, two or more contacts or other items such as conductive layers or components are said to be “electrically connected” if they are connected by a material which is sufficiently conducting to ensure that the electric potential at each of the contacts or other items is substantially the same or about the same regardless of bias conditions.
The device of FIG. 2 includes a single high-voltage E-mode transistor 21 which can be encased in the same or a similar package to the hybrid device of FIG. 1. The source electrode 41 of the high-voltage E-mode transistor 21 can be connected to the source lead 11, the gate electrode 42 of the high-voltage E-mode transistor 21 can be connected to the gate lead 12, and the drain electrode 43 of the high-voltage E-mode transistor 21 can be connected to the drain lead 13. The device in FIG. 1 and the device in FIG. 2 are both capable of blocking high voltages between the source lead 11 and drain lead 13 when 0V is applied to the gate lead 12 relative to the source lead 11, and both can conduct current from the drain lead 13 to the source lead 11 when a sufficiently positive voltage is applied to the gate lead 12 relative to the source lead 11.
While there are many applications in which the hybrid device of FIG. 1 can be used in place of the single high-voltage E-mode device of FIG. 2, acceptable levels of reliability for hybrid devices such as that of FIG. 1 have been difficult to obtain. Improvements in device design which lead to higher levels of reliability are thus desirable.